A chemically-defined tissue culture system developed for the BALB/MK epithelial cell line was used to study (1) how the introduction of activated viral oncogenes alters the cell line's growth factor requirements, and (2) what growth factors elicit a mitogenic response in this cell line and how these growth factors interact with each other. In this defined media system, uninfected BALB/MK keratinocytes required only two growth factors for growth--insulin and epidermal growth factor (EGF). Oncovirally infected BALB/MK demonstrated four patterns of growth factor requirements: (1) requirements unaltered from parental line (v-raf); (2) partial escape from EGF requirement (v-mos, v-fms, v-erbB); (3) complete escape from EGF requirement (v-K-ras, v-H-ras); and (4) escape from all growth factor requirements (v-fgr). Three of these viral infectants, v-K-ras, v-fgr, and v-fms, demonstrated release into the medium of EGF-like activity, presumably TGF alpha. No viral infectants produced insulin-like activity. In defined medium mitogenic assays, the following growth factors were found to be BALB/MK mitogens, in descending order of potency: acidic fibroblast growth factor (FGF), basic FGF, EGF, and insulin. Synergism was noted between insulin and EGF or insulin and basic FGF.